1. Field of the Invention
The invention relates to diazonium salt compounds and their use in diazotypy.
2. Brief Description of the Prior Art
The diazotype reproduction process is well-known to the art and is described in great detail in "Light Sensitive Systems" by Jaromir Kosar, John Wiley & Conc, Inc., New York 1962, and in "Reproduction Coating" by E. Jahoda, 4th Edition Andrews Paper & Chemical Co., Inc., Port Washington, N.Y.
In general, diazotype reproduction prints are obtained by an imagewise exposure of a light-sensitive diazo material to ultraviolet light followed by development of the exposed diazotype material. There are several different methods available to develop the latent image imposed on the light-sensitive diazo material by the light exposure including ammonia development, amine development, thermal development and moist (liquid) development.
The light-sensitive diazo material generally comprises a light-sensitive diazo composition affixed to a support base such as paper or film such as polyethylene terephthalate film, cellulose acetate, or a polymeric resin coated, like base support. The light-sensitive diazo composition may comprise a light-sensitive diazonium salt compound in admixture with diazo enhancing compounds. Upon exposure of the light-sensitive diazonium compound to ultraviolet light through a translucent original having opaque image portions, the unmasked portion of the diazonium salt are decomposed by the ultraviolet radiation whereas the masked portions are left undecomposed. The latent image created by the image-wise exposure may then be developed by the methods described above.
In the so-called "dry development" process, the light-sensitive diazo composition will have contained, in addition to the light-sensitive diazonium salt, an azo coupling agent or color former and an acidic coupling inhibitor. Development of the latent azo dye image is accomplished by placing the exposed diazo material in an alkaline atmosphere which neutralizes the acidic inhibitor, allowing the undecomposed diazonium salt and coupler to react. The latent image is thereby developed.
In the moist development process (also called the one-component process) the light sensitizing diazo composition contains essentially the diazonium salt compound. After image-wise exposure, development of the latent azo dye image is accomplished by applying to the light-exposed diazo material a buffered solution containing the azo coupling agent (which then reacts with the undecomposed diazonium salt to develop the latent image).
The prior art diazonium compounds employed in preparing light-sensitive diazo materials are generally used in the form of their acidic salts such as the zinc chloride, cadmium chloride, stannic chloride, sulfate and borofluoride salts. The acid salt derivatives are generally more stable than the parent diazonium compounds, which will often decompose spontaneously under ambient conditions.
For the preparation of diazo sensitized paper materials, the diazonium compound and ancilliary chemicals are dissolved in water and the resulting diazo composition is applied from the aqueous media to the base paper. It is, therefore, necessary for the diazonium salt to be substantially soluble in water. From a practical point of view, this requirement restricts the choice to a very few kinds of diazonium salts and in particular to the zinc chloride or the sulfate salts of the diazonium compound. Sulfate salts of diazonium are extremely soluble in water (and as a consequence are very difficult to isolate in solid form). Zinc chloride salts have a more limited solubility in water and are easy to isolate in solid form. However, in spite of the stabilizing effect of the zinc chloride, most diazonium chlorozincates are unstable when subjected to moderate heat. In addition such salts can be readily ignited and once ignited many burn vigorously with the emission of profuse toxic fumes and vapors.
For the preparation of diazo materials such as polymeric films and polymeric resin coated papers bearing diazo compositions, the diazonium and other ingredients are dissolved in polaric organic solvents such as alcohols, ketones or glycol ethers. The diazo composition is then applied from the solvent media to the base support material. It is therefore necessary for the diazonium salt to be soluble in such solvents. Borofluoride salts of diazoniums are generally soluble in organic solvents but most of them are, like the chlorozincates, unstable when subjected to moderate heat. They can also be readily ignited and when ignited burn vigorously with the emission of toxic fumes and vapors.
Because of the aforementioned serious hazards, special precautions and measures must be taken when handling or transporting the prior art diazonium salt compounds and diazo compositions. The Code of Federal Registrations, 49, Transportation, of Dec. 31, 1976 requires that any solid material which under conditions normally incident to transportation is liable to cause fire through friction, retained heat from manufacturing or processing or which can be ignited readily and when ignited burns so vigorously and persistently as to create a serious transportation hazard, be labelled "Flammable Solid" and be subjected to all the rules and regulations appertaining to the transportation of hazardous materials. In the case of international transportation, severe restrictions are imposed by the International Air Transport Association (IATA) for the air shipment of flammable solids, and by the Intergovernmental Maritime Consultive Organization (IMCO) for the ocean shipment of flammable solids. For instance, the IATA regulations requires a flammable solid to be packed in separate suitable containers of no more than 500 grams each with a total of no more than 12 kilograms. The IMCO regulations require that a flammable solid be stowed on the deck of a cargo ship, in a specially allocated area.
All the above mentioned regulations make the shipment and handling of prior art diazonium compositions not only more difficult, but also considerably more expensive than compounds which do not fall in a hazardous category.
In addition to the flammability hazards associated with diazonium chlorozincate salts, their relatively low thermal stability gives them a limited shelf life. Commercial diazonium chlorozincate salts tend to decompose slowly with time, even at room temperature, and in so doing lose part of their basic functional properties. In extreme cases of poor storage condition, for instance when a diazonium salt is exposed to tropical climate conditions, the decomposition can be so advanced as to make the diazonium totally unusable. For this reason it has often been necessary to keep diazonium compounds and compositions in refrigerated areas. This adds substantially to the cost of storage and imposes severe restrictions on inventory levels.
It has been suggested in order to decrease the hazard associated with unstable diazoniums to mix them with solid diluents or stabilizers such as tartaric acid, citric acid, aluminum sulfate, boric acid and other organic or inorganic salts. However, to obtain a sufficient reduction of the hazard, often up to 50% of diluents are required. Such a procedure reduces the rate of propagation of the flame, when the diazonium is ignited, but does not alter the flammability of the diazonium salt or its rate of thermal decomposition (which is inherent to the particular diazonium salt).
The diazonium salts of the present invention obviate many of the problems associated with the prior art diazonium compounds. The diazonium salts of the present invention exhibit a greater degree of thermal stability, non-inflammability and shelf-life. These improved characteristics are also found in diazotype materials prepared from the diazonium salts of the invention.
Other advantages of the compounds, compositions and diazotype reproducing materials of the invention will be described more fully hereinafter.